fp's assembly related confusion

hi to all
i was programming in assembly code in fragment program
i want to ask
let this code

PARAM c = {1.0,0.0,0.0,0.0};#Red
MOV result.color.xyzw c.xyzw;#Red to color

it give red color to the object where the progs call aplied
and in that code

i can punch this lines before MOV and after first line

PARAM a = {0.0,0.0,0.0,0.0};#all zerowed
PARAM b = {1.0,0.0,1.0,0.0};#not all zerowed

but if after that i put

MOV b,a;#b=a

then red color is not on object it was may removed/black in color
i want to ask in assembly code is there limit that
MOV(assignment) only done at once when second time seen it ignored
if the parameter not used are assigned then code not works.

thanks in advance
apologies for bad english and abused words